The danger beneath our feet

THREE million visitors a year explore the breathtaking scenery, wildlife and flora at Yellowstone National Park, one of the most popular attractions in the United States and the world's first national park. Eight kilometres beneath their feet, lies a boiling chamber of magma, containing the equivalent of 1,000 Hiroshima bombs.

This is one of around 40 supervolcanos across the world, whose eruptions, when they come, will cause devastation on a scale unknown in human history. But it is only recently that scientists have realised their destructive power - and the risk of an impending explosion. This weekend, a BBC drama looks at the impact an eruption could have.

Most of the supervolcanoes are long-extinct, including the ones at Glencoe in Scotland and the Peak District in England. But a handful remain active, including those at Yellowstone, Long Valley in eastern California, Phlegrean Fields, west of Naples in Italy and Taupo in New Zealand.

But the sleeping giants are not easy to detect. Unlike their smaller cousins, they are not lava spewing mountains, but are formed by depressions or huge collapsed craters, called calderas, which lie beneath the earth's surface.

The last supervolcano to erupt was at Toba, in Sumatra, Indonesia, about 74,000 years ago, sending 2,800 cubic km of magma spewing from the ground. It blew a hole in the Earth around 100km wide, and particles from the eruption stayed in the air for six years. Ice-core records show that it may have reduced global temperatures by as much as 10 degrees celcius.

But by far the largest supervolcano - 45 miles long by 30 miles wide and large enough to hold the world's biggest city, Tokyo - and the one with the most lethal potential, is underneath Yellowstone National Park. Yellowstone has experienced three supervolcanic eruptions in the past: around 2.1 million years ago, 1.3 million years ago and 640,000 years ago. Its regular eruption cycle of 600,000 years means some scientists believe we are overdue another one. They also believe the sleeping giant is breathing. The BBC says that volcanologists have been tracking the movement of the magma under the park and have calculated that in parts of Yellowstone, the ground has risen over 70cm this century. It could mean the harmless movement of lava, flowing from one part of the caldera to the other. It could also mean something much more sinister.

When Vesuvius erupted, Pompeii was submerged and at least 5,000 of its citizens were killed by a wave of red hot ash and gas, known as a pyroclastic flow. If Yellowstone were to erupt, the entire planet would feel its magnitude. It would be enough to destroy all life around the volcano , including Denver and Salt Lake City. The last time it erupted, it is said to have blasted 1,000 cubic kilometres of volcanic rock into the atmosphere, settling ash over more than half of the US.

BUT it is not just the US which would feel the effects. In Europe, summer would turn to winter almost overnight, with the UK experiencing a mini-ice age. It will become impossible to grow food, the sea would freeze in some places, and species unable to cope with the rapid change in temperature would die off.

South East Asia and sub-Saharan Africa would see vegetation die off and crops fail, as 90 per cent of sunlight was prevented from reaching the Earth by a cloud of sulphuric acid. Starvation is expected to claim the lives of around one billion people worldwide.

Says Professor John Davidson, head of earth sciences at Durham University: "If Yellowstone erupted, the volcanic products would be ejected 10/15,000 metres high, and what goes up must come down, as a thick blanket. The column of hot ash and gas would collapse and form a pyroclastic flow which would trail along at speeds of up to 150 km per hour.

"The ash itself would make the atmosphere pretty opaque for a while. It could certainly block out sunlight to the entire planet for a short period of time, maybe months, possibly more, and bring on a nuclear winter. In terms of gases, the sulphur dioxide would be a serious problem and would have a cooling effect on the climate."

Some scientists have predicted that the effects could last four or five years. They estimate that around 3,000 square miles of the US would be destroyed immediately, and the planet's eco-system would break down. But there is no way of telling when Yellowstone will blow. "It is easy to detect that it has happened and that there have been large eruptions there," says Prof Davidson. "So the volcanic history isn't a problem, it's one of the best ways of predicting volcanic behaviour.

"However, as the volcano gets bigger, its behaviour becomes less easy to predict because there are fewer eruptions. It's also difficult to detect what volume of magma there is. It could be left over from a previous eruption, but it's also possible it could have been created fairly recently."

And because scientists do not know how the volcanoes are going to behave, it is difficult to assess what, if any, warning signals there may be. Vulcanologists are also aware of the impact of their words if they get a prediction wrong.

Says Prof Davidson: "You can say there's an X per cent chance of an eruption in an X time, but how high is the percentage? If it is 90 per cent in the next 100 years, that's not a very useful prediction because it could come tomorrow. It's very difficult to know what we could do.

"Moving populations is a knee-jerk reaction, but in the case of something enormous, as this eruption potentially could be, it is really not that feasible, because such a large area would be devastated."

But just as Mount St Helens in Washington State had a number of small eruptions in the weeks before it blew in its entirety in 1980, there would be some signs to watch out for. They would include possible changes to the Earth's surface and increase in gases such as carbon dioxide and sulphur dioxide. Either way, Prof Davidson is not losing sleep over it.

"There's not a lot of point in being concerned about it because there's not a lot you can do about it," he says. "Volcanic eruptions take place all the time, they're fairly small and there's a pretty good relationship between the frequency that they erupt, and the size.

'THE biggest, supervolcanoes, erupt only once every few hundreds of thousands to millions of years. The likelihood of one occurring is pretty low."

He also believes, like the US Geological Survey (USGS), that Yellowstone could be the last supervolcano to blow. There is far greater concern over Long Valley in California, since 1980 when an earthquake swarm - a series of small earthquakes - struck the southern margin of the caldera. There have since been a number of earthquake swarms at Long Valley, which can indicate a movement of magma, and according to the USGS, a continued "dome-shaped uplift of the central section of the caldera, accompanied by changes in thermal springs and gas emissions".

Even if the warning signs were to come, there would be little the US Government could do, aside from mass evacuation. You may have a chance of blasting an Earth-bound asteroid out of the sky, but if a supervolcano were to erupt, there would be nothing to stop the planet being plunged into darkness and the temperature plummeting.

But even the prospect of a nuclear winter which we can do nothing about is not necessarily reason to panic, at least according to Prof Davidson. "I'm not alarmed by it," he says. "Would I go live there? Yes, it wouldn't bother me at all."

* Supervolcano is on BBC1 on Sunday and Monday, 9pm. Both parts are followed by a documentary, Supervolcano: The Truth About Yellowstone, on BBC2 at 10pm.